1. Field of the Invention
This invention relates to depolymerization processes which produce a product with lower molecular weight than the feed material. More particularly, this invention relates to the synthesis of cyclic esters from linear, branched, or higher molecular weight cyclic polyesters, especially those polyesters derivable from reaction of dicarboxylic acids with diols or from polymerization of monohydroxy monocarboxylic acids. Still more particularly, this invention is related to a process for synthesizing cyclic ethylene brassylate (also called "ethylene tridecanedioate"), cyclic ethylene dodecanedioate, and cyclopentadecanolide (also called "omega-pentadecalactone"), which are especially useful in perfumery.
2. Statement of Related Art
The general process of depolymerization is described by Mark et al. (editors), Encyclopedia of Polymer Science and Engineering, Volume 4, pages 719-745 (John Wiley & Sons, New York, 1986). A brief paragraph, with several literature citations, about the synthesis of cyclic compounds by depolymerization appears in this reference at the bottom of page 740.
This invention is particularly concerned with processes in which the product or products initially formed by depolymerization undergo further chemical reaction to produce the eventual desired product, especially a cyclic ester or lactone.
U.S. Pat. No. 4,709,058 of Nov. 24, 1987 to Cahill, Jr. et al. teaches the general primary art on which one specific embodiment of this invention is intended to improve. The entire specification of U.S. Pat. No. 4,709,058, except for any part that may be inconsistent with any explicit statement herein, is hereby incorporated herein by reference. The earlier prior art is reviewed in this Cahill patent.
In general terms, the Cahill patent teaches that the known reactions by which linear polyesters can be converted to cyclic compounds having from 8 to twenty carbon atoms in the cyclic ring are greatly improved for practical purposes when carried out in the presence of a polyolefin which is liquid but does not boil at the reaction temperature and which is chemically inert to the linear polyester feed material and the cyclic product at the temperature of the reaction.
The major teachings of Cahill are directed toward the benefits obtained by the presence of polyolefin during the reaction, and there is relatively little taught about mechanical reaction conditions. Cahill does teach the possibility of either batch or continuous reaction in general terms and gives at least one example of each, but in all cases the reactors taught by Cahill are stirred batch reactors.
In attempting to extend the processes taught by Cahill to longer reaction times than are explicitly taught therein, it has been found that a substantial amount of residue eventually accumulated in the reactor, and that during prolonged operation, the down time required for cleaning the reactor was at least half as great as the amount of useful operating time that could be achieved. A major object of this invention was to greatly increase the ratio of working time to time required for reactor cleaning in protracted operation of a process for producing cyclic compounds by decomposition of higher molecular weight polyesters.